Interpretive Summary: Russian thistle (Salsola tragus) is a summer annual broadleaf weed infesting arid and semiarid agricultural and non-cropland in more than 40 countries of the world. In the Pacific Northwest (PNW), USA, Russian thistle is an intense competitor, infests 60% of the wheat producing area, and is the main pest preventing the adoption or adaptation of alternative broadleaf crops. Most of the biological, ecological, and management studies of Russian thistle in agriculture settings have been conducted in the PNW. These studies found that Russian thistle is a severe problem in growing crops, after crop harvest, and during the summer fallow year. This weed has intense, rapid, and early spring root growth and can reduce wheat yield up to 55%. After crop harvest, Russian thistle produces an abundance of biomass and seed and can extract so much water from the soil that it may prohibit the production of next year’s spring crop. In the PNW, Russian thistle resistance to sulfonylurea herbicides was discovered. Based on all of these studies, effective Russian thistle management strategies have been developed for PNW growers. These strategies include when, why, and how to control (suppress) Russian thistle in all situations. The strategies optimize crop competition and the reduction of weed seed production. Current research information and growers’ feedback indicates that effective weed control, soil conservation practices, and farm profitability are compatible when waging war against this weed.

Technical Abstract:
One of the major broadleaf weed species infesting the arid and semi-arid regions of the world is Russian thistle (Salsola spp.). It is found in more than 40 countries in the world, infests an estimated 41 million ha in the western United States, and 1.8 million ha of land in the winter wheat-summer fallow region of the Pacific Northwest (PNW). In the PNW it costs growers more than $50 million annually in reduced crop yield, quality, and control measures. In addition, Russian thistle has impeded the production of no-till spring crops, especially alternative broadleaf crops, which have been produced to reduce the number of erosive fallow areas. Russian thistle thrives in this region because of its season-long germination and seedling establishment capability, its high water use efficiency, and its rapid and extensive root growth in early spring. After crop harvest, Russian thistle that either survived in-crop herbicides or became established after the crop was sprayed, will produce an abundance of biomass and seed, and will extract more than 100 liters of water from the soil. This soil moisture loss may be so great that soil recharge by winter and spring precipitation may not be sufficient to produce a crop the following year. Russian thistle management strategies must optimize crop competitiveness and focus on preventing seed production throughout the crop rotation cycle.